1. Field of the Invention
The present invention relates to a semiconductor laser and a method for manufacturing the same and, in particular, to a semiconductor laser having a compound semiconductor layer using magnesium (Mg) as a dopant or a constituent element.
2. Description of the Related Art
In recent years, a III-V Group compound semiconductor, such as GaAs, InP and GainAlP, can be grown, by a metal organic chemical vapor deposition (MOCVD), with better controllability and has been extensively utilized as a constituent component for a semiconductor laser and light emitting diode.
In the MOCVD method, Zn is generally used as a p-type dopant for the III-V Group compound semiconductor and, being used as a p-type dopant for the GaAs, exhibits a substantially good doping characteristic. If, however, Zn is used as a dopant for a P-containing III-V Group compound semiconductor, such as InP and GaInAlP, the percentage of Zn incorporated into the compound semiconductor is low and it has been difficult to dope the Zn in a desired amount. Further, the activation degree of the Zn is low and the diffusion of it in a layer is fast, resulting in poor controllability.
The elements Be and Mg for example may be considered as a p-type dopant in place of Zn. The element Be exhibits a good characteristic as a p-type dopant in a molecular beam epitaxy (MBE) method. However, the organic Be compound is strongly toxic and it is very difficult to use it as the dopant in the MOCVD method. On the other hand, a straight chain type organometal compound of Mg, such as dimethylmagnesium and diethylmagnesium, is not toxic in nature, but very strong in its self-association and never occurs in single form. For this reason, the Mg-containing straight chain type organometal compound is not suitable as a doping agent.
Recently, biscyclopentadienylmagnesium (Cp.sub.2 Mg) relatively high in vapor pressure has been used as a Mg-doping material. However, the material Cp.sub.2 Mg is deposited as a residual one in a crystal growing apparatus and exhibits a memory effect so that the doping control is very difficult. In spite of the fact that a three-orders-of-magnitude-greater concentration variation is required in a 0.1 .mu.m film thickness for a double heterostructure (DH) laser device, such a sharp concentration variation cannot be ensured at the present time. In order to enhance the vapor pressure, the methyl group-attached cyclopenta ring type material "bismethylcyclopentadienylmagnesium" [(CH.sub.3)Cp.sub.2 Mg] is known as one example of a dopant and, even in this case, no sharp Mg-concentration variation is obtained at a doped-to-undoped interface.
In order to reduce a resistivity of a predetermined semiconductor layer, more amount of Mg has to be doped as a p-type dopant. In order to obtain a resistance value of, for example, about 0.5 .OMEGA..multidot.cm to 10 106.multidot.cm, the Mg has to doped at a concentration amount of about 5.times.10.sup.18 /cm.sup.3 to 5.times.10.sup.19 /cm.sup.3. In the case where more amount of Mg is so doped, the laser performance is lowered and, for the case of a semiconductor laser having an active layer as narrow as below 5 nm in thickness in particular, a greater adverse influence is exerted over an operating current.
There are sometimes the cases where the Mg is employed not only as a p-type dopant for the III-V Group compound semiconductor but also as one of the constituent element of the compound semiconductor layer of II-VI Group compound semiconductors, etc. Even in this case, it has been difficult to effect high accurate control of an Mg composition, the reason of which is the same as set out above.
In order to incorporate Mg into the II-VI Group compound semiconductor, an amount of Mg is not less than 10% and preferable in terms of decrease in an operating current and shortening a wavelength. For a semiconductor laser principally of a II-VI Group compound semiconductor containing such an amount of Mg, it has been difficult to obtain adequate reliability. A semiconductor laser, having an active layer as thin as less than 5 nm in particular, is markedly lower in its performance, such as increasing an operating current.
In this way, the simple alkyl compounds of Mg, such as dimethylmagnesium and diethylmagnesium, are lower in vapor pressure and not suitable as an Mg feed in the growth of a compound semiconductor layer by the MOCVD method. For a Mg compound including a cyclopenta ring, it has been difficult to control the Mg composition, or Mg doping amount, in a compound semiconductor layer because a high memory effect is involved.
In the case where the Mg doping amount is increased so as to lower the resistance of a p-type cladding layer, a semiconductor laser principally of a Mg-doped III-V group semiconductor is lower in the characteristics, such as an efficiency of an emitting layer and, in the worst case, the operation is not possible. Even with a semiconductor laser principally of a II-VI Group compound semiconductor containing Mg as one of its constituent elements, if more Mg is introduced into a cladding layer, a similar problem arises therefrom.
In either case, if the Mg amount is increased in a semiconductor layer, the semiconductor laser is lower in its characteristics, thus failing to obtain adequate reliability.